Method of preparing low density wrought zinc alloy with improved strength and low temperature ductility

ABSTRACT

A METHOD OF HEAT TREATING A WROUGHT ALLOY CONSISTING ESSENTIALLY OF 18-30% ALUMINUM, UP TO 3% COPPER, UP TO 0.10% MAGNESIUM, UP TO 0.10% LITHIUM AND THE BALANCE ZINC APART FROM INCIDENTAL IMPURITIES, WHICH COMPRISES SLOW COOLING SAID ALLOY FROM BETWEEN ABOUT 380* C. AND ABOUT 290* C.

June 2 6, 1973 p, CHOLLET ETAL 3,741,819

METHOD OF PREPARING LOW DENSITY WROUGHT ZINC ALLOY WITH 1 7 IMPROVED STRENGTH AND LOW TEMPERATURE DUCTIL'I'Y Grigiual Filed Nov. 27, 1968 5 Sheets-Sheet 1 June 26, 1973 P. CHOLLET ET AL METHOD OF PREPARING LOW DENSITY WROUGHT ZINC ALLOY WITH IMPROVED STRENGTH AND LOW TEMPERATURE DUCTILTY Original Filed Nov. 27, 1968 5 Sheets-Sheet 3 992 gogous +05'c 400 sgugus kg ggrgcrolo E n: LU a E m 200 *3 l l l l l O 25 5O 75 I00 '25 I50 TIME, MINUTE F|G 4 INVENTDKS PIERRE Gym/.57

530a RR) GER vfi/s l2 rTm/vsys United States Patent Int. 01. 622i 1/16 US. Cl. 148-115 R 15 Claims ABSTRACT OF THE DISCLOSURE A method of heat treating a wrought alloy consisting essentially of 18-30% aluminum, up to 3% copper, up to 0.10% magnesium, up to 0.10% lithium and the balance zinc apart from incidental impurities, which comprises slow cooling said alloy from between about 380 C. and about 290 C.

This invention relates to a mehod of heat treating zinc aluminum alloys. The alloys of the present invention optionally contain copper and other alloying elements and can be used in the wrought heat treated condition. The heat treatment is also beneficial to subsequent cold working operations. This is a division of appliction Ser. No. 779,377 filed Nov. 27, 1968 and now abandoned.

Alloys presently used for the purposes contemplated for the alloys of this invention include those of the Zn-Cu-Ti type. However, these alloys sulfer from disadvantages such as poor bending, folding and impact properties at temperatures below room temperature. A further disadvantage is their high density as compared to Al and Al alloys. This latter disadvantage entails a higher cost to the Zn-Cu-Ti alloys on a volume basis despite a higher cost per pound for Al ingot. Still another disadvantage of the Zn-Cu-Ti type alloy is that attempts to increase the tensile strength above, say 30,000-35,000 lbs./ sq. in. usually result in a deterioration of the room and particularly the sub-normal temperature bending and folding properties With the wrought heat treated alloys of the present invention, the sub-normal temperature bending and impact properties can be improved over those of the commercial Zn-Cu-Ti type with the additional advantage that the tensile strength and hardness are higher. Without being limited to this explanation it is believed that the improvement of the sub-normal temperatures properties is due to the presence of a ductile, Al-rich phase in the structure. Furthermore, the measured density of the alloys of the present invention is about 23%33% lower than that of Zn-Cu-Ti type alloys, which makes them economically attractive on a volume basis.

Alloys having a composition such as that contemplated for the alloys of the present invention are broadly disclosed for example in U.S. Pat. 1,945,288 to Morell dated Jan. 30, 1934 and U.S. Pat. 1,767,011 to Pack dated June 24, 1930. Such alloys are generally known in the art as die casting alloys. Similar alloys have, however, been hot worked in the temperature range contemplated in the pres- "ice ent invention, as for example in U.S. Pat. 2,169,441 to Winter, Aug. 15, 1939, but Winter failed to teach the improvement in physical properties which can be achieved by a critical heat treatment following hot working. Winter also failed to teach that the cold working properties of these alloys can be improved significantly by an initial, critical heat treatment.

An object of this invention is to provide a method of heat treating the alloy to obtain the desired improved properties of the heat treated alloy.

By one aspect of this invention there is provided a wrought, heat treated alloy containing l8-30% aluminum, with the balance zinc which has a tensile strength in excess of about 23,000 lbs./in.

By a further aspect of this invention there is provided a wrought heat treated alloy, having a tensile strength of at least 29,000 lbs./in. containing 18-25% aluminum, 0.11% copper, 0-0.1% magnesium and 0-0.l% lithium and the balancing being essentially zinc and incidental impurities.

By another aspect of this invention there is provided a method of heat treating the alloys of the preesent invention wherein the alloys are slowly cooled from about 380 C. to yield a coarse grained lamellar structure, at an optimum cooling rate of about 15 C./min.

By yet another aspect of this invention there is provided a method of heat treating the present alloys wherein the alloys are slowly cooled from 350 C. to yield a less coarse grained lamellar structure, at an optimum cooling rate of about 1.5 C./mm.

By yet another aspect of this invention there is provided a method of heat treating the present alloys wherein the alloys are slowly cooled from 290 C., to yield a fine grained structure, at an optimum cooling rate of about 15 C./min.

By yet another aspect of this invention there is provided a method for obtaining the maximum inverse creep rate from the present alloys wherein the alloys are cooled at an optimum cooling rate of 1.5" C./min. from about 380 C.

The invention also provides a method of producing a wrought heat treated zinc base alloy containing at least 18% and up to about 30% by weight of aluminum and optionally at least one of the elements copper, in an amount up to 3%, magnesium, in an amount up to 0.10% and lithium, in an amount up to 0.10% which method comprises hot working the alloy by rolling or extrusion and cooling the hot worked alloy from a temperature not greater than the solidus of the particular alloy at a rate not exceeding 2 C. per minute.

With advantage, the cooling rate is not less than about 1.0 C./min. and the most preferred cooling rate is about 1.5 C./min.

It is preferred to slowly cool the alloy trom a temperature of about 380 C. to the eutectoid transformation temperature in about 50l00 minutes or from a temperature of about 350 C. to the eutectoid transformation temperature in about 40-70 minutes or from about 290 C. to the eutectoid temperature in about 20-50 minutes.

Throughout this specification the term slowly cooled is used to mean that the samples are cooled in an air circulating furnace with the power and fan OE and with the door closed. Percentages of alloying constituents are to be construed as percentages by weight throughout this specification.

Although an upper temperature of about 380 C. is taught herein, it will be appreciated that the solidus of some alloys falling within the scope of this invention may be just above or below 380 C. and in these cases the maximum temperature from which slow cooling may be effected is the solidus temperature.

It will be appreciated that the preferred alloys of this invention are ternary and quaternary alloys, but that the binary ZnAl alloys may also be heat treated by the method of the present invention. The binary Zn-22% Al alloy is well known to those skilled in the art as a super plastic alloy, and further more one which has a low tensile strength, is strain rate sensitive, and is unstable insofar as it is susceptible to aging during which process there is an initial improvement in tensile properties followed by a considerable degradation. Wrought binary alloys heat treated by the method of the present invention have improved tensile properties and are stable.

Some embodiments of the invention will now be described by way of example, reference being made to the accompanying drawings in which:

FIG. 1 is a photomicrograph at a magnification of X400 of a longitudinal cross section of a wrought Zn- 20%-Al-1% Cu alloy slowly cooled from 380 C. according to the present invention.

FIG. 2 is a photomicrograph at a magnification of X400 of a longitudinal cross-section of the wrought alloy of FIG. 1 but slowly cooled from 350 C.

FIG. 3 is a graph of creep test results for a Zn-20%- Al-1% Cu alloy showing the effect of different heat treatment temperatures.

FIG. 4 is a graph showing various cooling rate curves from 380 C. for a Zn-20%-Al-l% Cu alloy rolled at 350 C.

The composition in Weight percent of the alloys contemplated for the persent invention falls in the following range:

Percent Aluminum l8-30. Copper Up to 3. Magnesium Up to 0.10. Lithium Up to 0.10. Zinc-l-incidental impurities Balance.

An alternative composition in weight percent of the alloys contemplated for the present inpention falls in the following range:

A preferred range for applications whereby ductility is the more important consideration is:

Percent Aluminum 18-25. Copper 0.1-l. Zinc+incidental impurities Balance.

A more preferred range for ductility applications is:

Percent Aluminum 18-22. Copper 1. Zinc-i-incidental impurities Balance.

A preferred range for applications Where creep resistance is the more important consideration is:

Percent Aluminum 18.25. Copper 0.1-1.

4 At least one of:

Lithium, positive amounts Up to 0.1. and;

Lithium, positive amounts Up to 0.1

As will be appreciated by those skilled in the art, lithium may be a replacement for magnesium or may be in addition to magnesium depending on the desired end use of the alloy.

The essential alloying element is aluminum which should be present in an amount of at least 18% in order to ensure that the alloy falls within the a phase of the Zn-Al phase diagram when heated at certain temperatures between the eutectoid temperature (275 C.) and the solidus line.

In circumstances where it is necessary to obtain a creep strength, in wrought sheet or plate alloys, comparable to those of the Zn-Cu-Ti type alloys, copper may be added in amounts up to about 1.0% and the aluminum content should, in such cases, be about 25%. The addition of copper also increases the tensile strength without decreasing the bending values.

The quaternary addition of Mg in amounts up to 0.10% in alloys containing Zn, Al and Cu results in an improvement of the inverse creep rate, an increase in the tensile strength and a decrease in the bending properties. Lithium may be substituted for magnesium in the quaternary alloy and is especially beneficial to the creep strength. Lithium is in fact, superior to Mg in this respect.

Magnesium is a particularly useful hardening element for those alloys which are used in the cold drawn condition. Alloys which do not contain Mg tend to soften with time when cold drawn after extrusion and/or heat treatment due at least in part to a recovery phenomenon and this tendency may be substantially eliminated by a Mg addition in the alloy. Similarly, Mg tends to prevent a heat treated alloy from softening with increasing amounts of total Wire drawing reduction for the same reasons.

By way of example, which should not be construed in a limiting sense, specific alloys which have been treated according to the present invention are:

(i) Zn-20% Al (ii) Zn-20% Al-O.1% Mg (iii) Zn20% Al 0.05% Li (iv) Zn-20% All.0% Cu (v) Zn-20% All.0% Cu-0.05% Li (vi) Zn-25% All.0% Cu (vii) Zn-25% Al (viii) Zn-20% All.0% Cu-O.1% Mg FIG. 1 shows a ph'otomicrograph at X400 magnification of a longitudinal cross section of the alloy of Example (iv) rolled at 350 C. and slowly cooled in a furnace from 380 C. and etched electrolytically in a solution including ethanol, perchloric acid and water. The structure is a typical ncar-eutectoid structure and is essentially lamellar, the white or grey background being the zinc-rich phase, whereas the lamellae consist of a mixture of the dark Al-rich phase and the white Zn-rich phase.

FIG. 2 shows a photomicrograph at X400 of the same alloy depicted in FIG. 1 but which has been rolled at 350 C. and slowly cooled in a furnace from 350 C. It is clear from a comparison of FIGS. 1 and 2 that cooling from 380 C. results in a much coarser grain structure than cooling from 350 C. The coarser grain structure gives the higher inverse creep rate.

As previously indicated, FIG. 3 represents a plot of the total creep deformation in percent as a function of time at 23:2 C. under a nominal stress of 10,000 lbs/in. for an alloy which has been heat treated at 350 C. and 380 C. Curve 1 represents the alloy rolled at 350 C. followed by slow cooling from 350 C. and curve 2 represents the alloy rolled at 350 C. followed by slow cooling from 380 C. It will be noted that furnace cool- 5 6 ing from 380 C. results in a room temperature inverse the eutectoid transformation temperature of 275 C. is creep rate many times greater than that of the same alloy reached in 40-70 minutes, and the optimum cooling rate slowly cooled from 350 C. The diiference is even greater from 380 C. is one wherein the eutectoid transformafor alloys cooled from 290 C. Similar tests to that detion temperature is reached in 50-100 minutes.

scribed for in FIG. 3 were conducted on alloys containing 5 Table I (below) shows the mechanical properties up to 25% Al, 1% Cu with and without magnesium or achieved by slow cooling from 380 C. following rolling lithium additions and in all cases the room temperature at 350 C. and it will be noted that there is remarkably inverse creep rate of the alloy slowly cooled from 380 little fall off in bending ductility at C. and that the was higher than that of the same alloy when cooled from tensile strength is well in excess of 30,000 lbs/in? with 350 C. or 290 C. 10 the single exception of the binary Zn-18%1Al alloy which it was also found that, under a given applied stress, is at the lower limit of the range of the alloys contemthe room temperature inverse creep rate of alloys of the plated for treatment. Even the binary alloys heat treated present invention rolled at 350 C. and furnace cooled by this treatment have tensile strengths considerably in from 380 C. compared favourably with the inverse excess of the minimum tensile strength (23,000 lbs/sq.

creep rate of the difierent commercial alloys which are 15 in.) contemplated for alloys of this invention.

TABLE I.-MECHANICAL PROPERTIES OF ROLLED Zn-Al ALLOYS SLOWLY COOLED FROM 380 C. AFTER ROLLING AT 350 C.

Bending values No. of 90 bends Tensile properties Room Vickers U.T.S., Elontem- Hardness lbs./ gation, pera- Number (10 Composition in. percent ture 0 C. kg.-load) 1 28,200 21 5-0 4-5 67-08 is iii-0.1% Cu 31,200 13 6-7 0-7 72-73 18% 010.25% Cu ,800 21 (5-7 4-5 74-75 Al-1.0% Cu- 35,300 42 7 4-5 83-84 Al 32,500 4-7 4-5 73-74 0.101% Cu--- 35, 400 13 4-5 3-4 76-77 20% A10.25% Cu. 36,000 10 5-7 4-5 81-82 20% Al-1.0% Cu- 35, 000 0-0 5-7 77-89 20 Al-2.0% Cu 40,000 23 5-0 4-5 01-02 20% Al-3.0% Cu. 43, 500 25 4-6 4-5 03-04 22% 000.1% Cu- 37, 200 11 a4 2a 82 22% A10.25% 011-- 38,000 12 4-5 3-4 85 22% Al-0.5% Cu 38,500 15 0-7 4-5 84 22% Al-1.0% Cu 13 0-7 4-5 89-90 25% Al-1.0% Cu--- 40,700 18 0 4 04-05 309' A1-1.0% Cu- 40, 000 27 5 4 92-98 20% A10.05% Li- 40,000 10 3 3 00-01 20 010.1% 41,800 15 5 4 20% Al-1.0% 011005 7 Li 43,000 18 4-5 3-4 05 20% iii-1.0% (Du-0.08 0 Mg 42,000 20 3 currently used for the same purposes as contemplated for The mechanical properties achieved by slow cooling from the novel heat treated alloys of the present invention. 40 350 C. and 290 C. following rolling at 350 C. are

FIG. 4 shows the cooling curves for a Zn-2'0%Al-1% similar to those shown in Table I above. Cu alloy and the approximate phase transformation tem- In order that a comparison between the alloys heat peratures for the binary Zn-20X Al alloy so that the treated according to the present invention and the comeifect of cooling rate can be compared to the phases mercial alloys used for similar application may be made, produced and so that importance of the eutectoid may similar property determinations were made on two combe appreciated. mercially accepted alloys. The two samples tested were The addition of Cu to the binary alloy afiects the phase obtained from two independent sources, and it will be transformation temperatures only slightly so that it is noted from Table H below that they had widely diifering permissible to consider the ternary cooling curves with properties. It is particularly important to note that the the binary phase transformation temperatures on a comtensile strength is, in each case, considerably lower than parative basis only. Curve (a) represents rapid cooling of that achieved in the Zn-Al-Cu alloys and that although about 70 C./min. as by air cooling, curve (b) represents the room temperature ductility is as good as, or even cooling of about 5 C./min. as by accelerated furnace better than the Zn-Al-Cu alloys this property falls off very cooling, curve (0) as depicted by the hatched area of the rapidly and at 0 C. is far inferior to that obtained with graph represents the preferred cooling rate of about 1-2. the Zn-Al-Cu alloys of the present invention.

TABLE II.MECHANICAL PROPERTIES OF COMMERCIALLY PRODUCED Zn-Cu-Ti ALLOYS USED AS STANDARD FOR COMPARISON WITH THE ALLOYS TREATED ACCORDING TO THE PRESENT INVENTION Bending values number Tensile properties of 90 bends Elon- Room Vickers U.T.S. gatem- Hardness lbs. tron, pera- Number Alloy identification Condition in. percent ture 0 C. (10 kg.-load) 1st Zn-Cu-Ti alloy As supplied, i.e., cold rolled and heat treated 21,500 30 12-15 2-3 58 Idem Rolled at 300 C. on laboratory mill 22,900 46 9-10 1-2 71-72 211d Zn-Cu-Ti alloy As supplied 23, 000 39 10-11 2-3 C./min. as in a furnace cool, and curve (d) represents Table III even more clearly shows the markedly sua y 310W 6001 of about as achieved y a perior low temperature properties of the present heat retarded furnace cool 7 treated alloys as compared to commercial alloys. The im- In order to obtain the best combination of mechanical propertifis cooling from C. is satisfactory but in pact strength is far superior and, even more important,

order to achieve the best creep properties (the highest the transition temperature of the alloys is inverse creep rate) cooling from 380 C. is necessary. substantially lowered in comparison with the Zn-Cu-Ti The optimum cooling rate from 350 C. is one wherein 75 alloys.

TABLE III.-COMPARISON OF SUB-STANDARD CHARPY V-NOTCH IMPACT PROPERTIES OF TWO ALLOYS OF THE PRESENT INVENTION AND A COMMERCIAL Zn-Cu-Tl TYPE ALLOY Impact properties Energy absorbed (tt.-lbs.)

Room Transition temtemperapere- Oomposition Condition ture, 0. ture C.

Zn-20%, Al-0.1% Cu l Rolled at 290 0., slowly cooled from 290 C 4. 3 2.4

Zmwy AH 07 Cu {Rolled at 350 0., slowly cooled from 380 C-.. 18 2. 8 1.7

Rolled at 350 0., slowly cooled from 350 C. 10 4. 1 2.3

Cold rolled 57 1. 8 Y 0. 5

1st Zn-Cu-Ti alloy Rolled at 200 C. 63 1.1 0. 5 Rolled at 300 C. 65 0. 9 0.5

The tensile tests were carried out on a Tinius-Olscn machine on standard ASTM test samples at a cross head separation rate of 0.2 in./min. The percent elongation was determined on a two inch gauge length using 0.027 to 2 0.028 inch thick sheet samples. The bend test was carried out as follows: the lower portion of a 0.75 in. wide strip was clamped firmly between a pair of dies having a radius of curvature of about twice the thickness of the sample, i.e. 0.54 in. The upper free portion of the sample strip was bent without tension back and forth from the vertical to a total angle of 180, and the number of 90 bends to fracture was counted. For the test at 0 C. the bending apparatus was immersed in an insulating box filled with ice and water.

The impact tests were carried out on a standard Charpy pendulum using sub-standard V-notched Charpy specimens. It must be appreciated that this test on notched specimens is very severe and that the resulting impact values cannot be compared with those which would have been obtained on standard V-notches and on unnotch'ed specimens. The transition temperature was defined as the temperature corresponding to max. min. 2

where E and E are respectively the maximum and minimum energy values. The temperature range investigated was from 30 C. to 150 C.

In summary, it is clear from the results tabulated in Table I that for the rolled alloys furnace cooled from 380 C. the tensile strength is about 50-75% higher than that of the two Zn-Cu-Ti alloy samples tested and tabulated in Table II. The tensile strength for the rolled alloys furnace cooled from 350 C. and 290 C. is also much higher than that for the commercial Zn-Cu-Ti alloys.

The relatively high values obtained at 0 C. in the bend test for the rolled heat treated Zn-Al-Cu alloy as compared with those obtained for the Zn-Cu-Ti alloy indicate the superior low temperature bending ductility of the Zn-Al-Cu alloys. The impact properties corroborate these results both in terms of ft.-lbs. of energy absorbed at room temperature as well as in terms of the transition temperature. As is well known to those skilled in the art, the lower the transition temperature, the better the low temperature ductility becomes, and the higher the absorbed energy, the tougher the alloy is.

The improvement in low temperature ductility brought about by the alloys heat treated according to the present invention is particularly important in applications in architectural fields and may prove significantly useful as in automobile trim. The wrought alloy heat treated according to the present invention and in particular a Zn-%Al-l% Cu alloy, have been chromium plated without any difiiculty using a standard commercial procedure for zinc die casting alloys. A sheet sample could be bent through 180 without any gross breaking away of the coating.

The present invention has thus far been discussed principally with reefrence to hot rolled alloys which have been furnace cooled, and it will be appreciated that ex- 5 at 350 C. Similarly tests show that Zn-20%-Al-1% Cu alloys, with and without magnesium additions may be cold drawn more advantageously if they are heat treated by slow cooling from 380 C. or even 290 C. before cold drawing. The results of such tests are summarized in Tables IV and V below.

TABLE IV.-MAXIMUM TOTAL REDUCTION Furnace Furnace cooled cooled .As exfrom from Nominal composition weight truded, 290 0., 380 0., percent percent percent percent Zn-20%, Al l% Cu 1 67.4(20) 93 (3020) 92 (20) 98 0-20) 90(20) 97(30-20-10) Zn-20%, lei-1%, (Jo-0.01%, Mg 4e (20) (30-20) Zn-20%, Ill-1%, Cu-0.09%, Mg 46(10-15) 70(20) 87(20) 1 Number in parenthesis indicates the amount of reduction per pass 1 TABLE V.M.AXIMUM REDUCTION PER PASS Furnace Furnace cooled cooled As exfrom from Nominal composition weight trudcd, 290 0., 380 0., percent percent percent percent Zn- Al-1%, Cu 20 35 35 Zn-20 0, 1.14%, (Eu-0.01%, M 20 30-35 211-2 iii-1%, ouooe, Mg 10-15 32 1 20 1 Only tried.

From the tests, it may be concluded that much larger total drawing reductions are possible from heat treated rods than from those drawn immediately after hot extrusion. The examples given above pertain to drawing speeds of 90 ft./min. using mineral oil as a lubricant. The temperature from which the rods are slow cooled is not critical but the examples cited above indicate that cooling from 380 C. is slightly superior to cooling from 290 C. In addition to the larger total reduction obtainable it is also possible to increase the amount of reduction per pass by approximately 15% when the alloy is slow cooled from 380 C. or even 290 C. before drawing.

We claim:

1. A method of heat treating a wrought alloy consisting essentially of l8-30% aluminum, up to 3% copper, up to 0.10% magnesium, up to 0.10% lithium and the balance zinc apart from incidental impurities, which comprises slow cooling said alloy from between about 380 C. and about 290 C.

2. A method as claimed in claim 1 wherein said alloy contains 18-25 Al, 0.l1% Cu and the balance essentially Zn.

3. A method as claimed in claim 1, wherein said alloy contains 20% A1, 1.0% Cu and the balance essentially Zn.

4. A method as claimed in claim 1 wherein said alloy contains 20% Al, 1.0% Cu, 0.08% Mg and the balance essentially Zn.

5. A method of heat treating a wrought alloy consisting essentially of 18-30% aluminum, and positive amounts of at least one of the following: up to 3% copper, up to 0.10% magnesium, up to 0.10% lithium and the balance zinc apart from incidental impurities, which comprises slow cooling the alloy from between about 380 C. and about 290 C. at a cooling rate between 1 C./min. and 2 C./min.

6. A method as claimed in claim 4 wherein said cooling rate is 1.5 C./min.

7. A method of heat treating a wrought alloy consisting essentially of 18-30% aluminum, at least one of the following: up to 3% copper, up to 0.10% magnesium, up to 0.10% lithium and the balance zinc apart from incidental impurities which comprises slow cooling the alloy from about 380 C. to ensure the formation of a coarse grained lamellar structure.

8. A method of heat treating a wrought alloy consisting essentially of 18-30% aluminum, at least one of the following: up to 3% copper, up to 0.10% magnesium, up to 0.10% lithium and the balance zinc apart from incidental impurities which comprises slow cooling the alloy from about 350 C. to ensure the formation of a. fine grained lamellar structure.

9. A method of heat treating a wrought alloy consisting essentially of 18-30% aluminum, at least one of the following: up to 3% copper, up to 0.10% magnesium, up to 0.10% lithium and the balance zinc apart from incidental impurities which comprises slow cooling the alloy from about 290 C. to ensure the formation of a finegrained structure.

10. A method as claimed in claim 7 wherein said alloy is hot rolled at about 350 C. before said slow cooling. 11. A method as claimed in claim 8 wherein said alloy is hot rolled at about 350 C. before said slow cooling. 12. A method as claimed in claim 9 wherein said alloy is hot rolled at about 350 C. before said slow cooling. 13. A method as in claim 7 wherein said alloy is hot rolled at about 350 C. before said slow cooling and subsequently cold worked after said slow cooling step.

14. A method as claimed in claim 8 wherein said alloy is hot rolled at about 350 C. before said slow cooling and subsequently cold worked after said slow cooling step. 1'5. A method as claimed in claim 9 wherein said alloy is hot rolled at about 350 C. before said slow cooling and subsequently cold worked after said slow cooling step.

References Cited UNITED STATES PATENTS 1,945,288 1/1934 Morell -178 AM 2,008,529 7/ 1935 Werley 75-178 AM 2,102,869 12/1937 Winter 75-178 AM 3,676,115 7/1972 Hare et a1. 1481l.5 R

FOREIGN PATENTS 119,486 10/1918 Great Britain 75-178 AM 512,758 11/1937 Great Britain 75-178 AM 335,270 2/1936 Italy 75-178 AM WAYLAND w. STALLARD, Primary Examiner US. Cl. X.R. 75-178 AM 

